Method of forming chemically bonded agglomerates

ABSTRACT

Thermally shock-resistant chemically bonded agglomerates suitable for use as a combustible fuel or in a gasification process are produced by: 
     (a) mixing particulate organic material comprising particulate carbonaceous fuel and/or animal excrement (such as dewatered comminuted sewage sludge) with a lignosulfonate, in the presence of water, with application of heat sufficient to maintain the lignosulfonate in fluid form, so as to deposit a film of lignosulfonate over the surface of the particulate material; 
     (b) oxidatively conditioning the lignosulfonate coated particulate material under conditions permitting escape of water present in the mixed particulate material, the conditioning either being in the presence of calcium ions or being followed by mixing the conditioned particulate material with lime; and 
     (c) shaping the resulting free-flowing plastic mixture under pressure, to form the thermally shock-resistant agglomerates.

BACKGROUND OF THE INVENTION

The present invention is concerned with agglomerates obtained fromorganic waste (such as sewage sludge solids or refuse derived fuel),which are suitable for use in a gasification process or as a combustiblefuel for use in a boiler or the like.

Sewage sludge is the insoluble part of raw sewage, which may be presentin the residue left after aerobic or anaerobic treatment of sewage;current methods of disposal of sewage sludge include landfill anddumping at sea, neither of which is satisfactory from an environmentalpoint of view. Difficulties are encountered with incineration of sewagesludge, because of the high water content of sewage sludge, the presenceof toxic heavy metals, and the fact that safe incineration would requireflame temperatures in the region of plasma temperatures.

However, various processes have been recently proposed for theproduction of fuel gas from waste materials, including sewage sludge.Typical such processes, for example, as disclosed in U.S. Pat. No.5,125,931 and WO94/17161, involve mixing dewatered sewage sludge withcrushed coal and comminuted cellulosic waste solids in defined ratiosand water contents, pressing the resulting mixture into briquettes orpellets, and gasifying the briquette by reaction with oxygen and steamto produce a mixture of hydrogen and carbon monoxide.

Among the binders disclosed for such briquettes are molasses, blackliquor, lignin sulfonate derivatives, brewery wastes, starch wastes,bentonite, slaked lime, unslaked lime, bitumen and pitch. No indicationof preference is given for any of these binders, and no conditions aregiven for use of these binders.

Briquettes based on organic waste materials, such as those describedabove, may be gasified to produce a fuel gas, which can be burnt in agas turbine, followed by exhaust gas recovery to generate steam, whichin turn can be used to drive a steam turbine, both turbines beingconnected to generators to generate electricity.

For satisfactory commercial use in such a gasification process, thebriquettes must meet the following criteria:

(a) they must have sufficient physical strength (cold green strength),in terms of crush strength, shatter resistance and abrasion/attritionresistance, for them to survive transportation, handling and feeding tothe gasification process without disintegrating;

(b) they must maintain their structural integrity in the gasifier asthey flow down the fuel bed under gravity (that is, they should notdisintegrate or form dust as they pass through the various reactionzones of the gasifier, including the drying, devolatilisation, pyrolysisand gasification zones);

(c) they should be resistant to thermal shock (that is, they should becapable of withstanding instantaneous exposure to high temperatures,which, for demonstration purposes, may be those in an oven at about 800degrees Celsius, without disintegration or significant loss ofstructural integrity);

(d) they should have good carbonization properties, that is, they shouldbe amenable to gasification;

(e) they should not stick together, in order to avoid blocking of thegas flow and to ensure maintenance of bed voidage (the inherent porosityof the briquettes themselves is also helpful in the various reactionsthey undergo in the gasifier); and

(f) in the case of a slagging gasifier, the ash content should form amolten slag pool, which can be tapped intermittently and quenched togive a solid vitreous frit which encapsulates the heavy metals and thelike.

The above criteria must be met within the financial constraints of acommercially viable process.

We have now devised a method of making improved briquettes and otheragglomerates, which involves the use of a specially selected class ofbinder, such that the resulting agglomerates readily meet the abovecriteria, and are selectively suitable either for use in a gasificationprocess as described above, or in a combustion process.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a method offorming thermally shock-resistant agglomerates each comprising achemically bonded macromolecular structure, which method comprises:

(a) mixing particulate organic material comprising a carbonaceousparticulate fuel and/or animal excrement with a lignosulfonate, in thepresence of water, with application of heat sufficient to maintain thelignosulfonate in fluid form, so as to deposit a film of lignosulfonateover the surface of the particulate material;

(b) conditioning the lignosulfonate coated particulate material withoxygen (for example, in an atmosphere of air) under conditionspermitting escape of water present in the mixed particulate material,said conditioning either being in the presence of calcium ions or beingfollowed by mixing the conditioned particulate material with lime(calcium oxide or calcium hydroxide); and

(c) shaping the resulting free-flowing plastic mixture under pressure,to form agglomerates.

The resulting agglomerates have a polymerised or macromolecular (pseudocoal-like) structure with excellent carbonization properties and thermalshock resistance.

It is known that phenol has reaction properties which affect thecarbonization properties of coal. Lignin contains substituted phenolunits and has similar beneficial properties on the carbonizationproperties of coal or the like.

It is believed that the thermal shock resistance of the agglomerates isbeneficially associated with the formation of sulfonyl and sulfurbridges between the macromolecular (lignin-based) backbones. Calciumsulfate formed from the calcium ions present is also believed tobeneficially contribute to such thermal shock resistance. The sulfurpresent in the resulting agglomerates is stoichiometrically bonded insuch a form that disadvantageous evolution of oxides of sulfur, or othernoxious sulfur compounds, is substantially precluded.

If the lignosulfonate is permitted to react with urea or a ureaderivative, this may contribute to the plasticity of the resulting mix;it is particularly preferred to supply urea or such a urea derivative byusing (according to a preferred embodiment of the invention), sewagesludge as the animal excrement. This reaction of lignin with urea orurea derivatives is a reaction which is known per se.

When sewage sludge is used, it is preferably initially screened anddewatered, typically to a water content of less than about 70% by weight(or a solids content of greater than 30% by weight). Other possibletypes of animal excrement which may be used according to the inventioninclude pig slurry, which is also preferably dewatered to a solidscontent such as that described above with reference to sewage sludge.

The use in the method according to the invention of a lignosulfonate,which is a material typically produced as a by-product in a paper-makingprocess, results in agglomerates of exceptional strength, as evidencedby their substantial resistance to impact, attrition and thermal shock.

The lignosulfonate is preferably present in a minor amount, such as upto about 15% by weight, of the mix used to form agglomerates, the amountreferred to relating to lignosulfonate in aqueous form.

The major influence on the mechanical properties of agglomerates, suchas briquettes, is the bonding between particles. On heating,carbonization, combustion or gasification of agglomerates formedaccording to the invention, the chemical bonding between the particlesextends through the structure of the particles, thereby establishing ametamorphic change throughout the agglomerate molecular structure. Thenature of this bonding is important not only under test conditions, butalso under service conditions.

If the bonding were to be of a purely adhesive nature, there would be aseparate phase at the interfaces between particles; according to theinvention, the bonding is chemical in nature and extends throughout themacromolecular structure of the agglomerate.

In the agglomerates produced according to the present invention, theparticles of the organic particulate materials, generally comprisingsewage sludge, become fused and chemically bonded to one another in sucha manner that the particles and lignosulfonate binder no longer remainas distinct separate entities or separate phases, such that undergasification conditions, the whole agglomerate has high green strengthand acts as a single homogeneous mass.

The lignosulfonate may be used in aqueous form, such as an aqueoussolution. A preferred aqueous solution contains 40 to 60% by weight (ona dry basis) of the lignosulfonate. Alternatively, when the remainder ofthe mix is itself aqueous, the lignosulfonate may be in solid orpseudo-solid form.

The lignosulfonate may include any suitable cation; examples of suitablecations include ammonium, sodium, calcium and magnesium. Calciumlignosulfonate is preferred because of the advantageous thermalshock-resistance of the resulting agglomerates, and the ability ofcalcium lignosulfonate to polymerise to provide agglomerates withexcellent green strength. Furthermore, when calcium lignosulfonate isused, it may not be necessary to treat the conditioned mix with lime, asthere may already be sufficient calcium ions present.

It is particularly preferred that the lignosulfonate should besubstantially sugar-free (sugars and the like having been removed,typically by fermentation or the like).

When lime (calcium oxide) is used, and/or when the lignosulfonate is inthe form of the calcium salt, the calcium ions have several effects, asfollows:

1. The calcium ions cause separation of lignosulfonate into α and βforms, which are defined according to the solubility of the lignin in abisulfite solution. The a lignins produce lignosulfonates which are lessheavily sulfonated than β lignins, and which are therefore moreinsoluble, when subjected to the same conditions. These more insoluble(less soluble) lignin/lignosulfonate complexes can be precipitated out.

2. Calcium oxide will, on being slaked to Ca(OH)₂, take up freemoisture. This reaction is sometimes important in that it removes freewater from the general green mix. Free water and thermal shockresistance are not compatible and the removal of water significantlyimproves the thermal shock resistance of the product.

3. Calcium will react with sulfur-containing compounds to produce CaSO₄.CaSO₄ in a melt situation (such as in a slagging gasifier) hasbeneficial effects on the viscosity of the actual melts themselves.

4. Calcium, in the presence of carbonate-forming materials, has acatalytic effect on various gasification reactions, resulting in anincreased quantity of methane formed.

When a carbonaceous fuel is present in the mix used according to theinvention, the ratio of sewage sludge to carbonaceous fuel may be, forexample, in the range 0.25 to 2.5:1. In some preferred embodiments, theamount of sewage sludge is greater than that of the carbonaceous fuel (apreferred ratio being in the range 1.5 to 2.5:1), which enables highquality agglomerates to be obtained, from low grade starting materialsso as to be suitable for supply to, for example, a gasification process.Gas resulting from such a gasification process appears to be cleanerthan gas from a conventional gasification process.

When sewage sludge is used, it should, as indicated above, be dewatered,and preferably also dried, prior to mixing with the lignosulfonate,typically to a water content of 10% by weight or less. The sewage sludgeand (where used) the carbonaceous fuel are in particulate form,typically in the form of short fibres, flock, flakes, powder, granulesor the like with a mean particle size of not more than 10 mm, morepreferably not more than 6 mm.

The agglomerate mix used according to the invention is preferably, in atleast some embodiments of the invention, substantially free ofrefuse-derived fuel or other materials derived from municipal refuse.Refuse-derived fuel can, in some cases, detract from the properties,such as the physical strength and thermal shock resistance, of theresulting agglomerates.

When a carbonaceous fuel is used in the agglomerates according to theinvention, the fuel may be any suitable particulate material, such asrefuse-derived fuel, petroleum coke, anthracite, bituminous orsub-bituminous coal, coke (from coal), coking coal, peat or lignite. Thefuel typically has a particle size in the range indicated above for thesewage sludge; a preferred particle size for the carbonaceous fuel isnot more than 2 mm.

When lime is used in the method according to the invention, it istypically in the form of the oxide (which is preferred when the mix hasa relatively high water content, such as about 5% by weight or more),or, in some cases, in the form of the hydroxide (slaked lime).

The agglomerates can be shaped into, for example, briquettes (formedbetween shaped dies or the like), pellets, similar bodies formed byextrusion, pan agglomeration or the like. The mix may be shaped toagglomerates by any suitable means, such as for example, in a ring-rollpress. The briquettes are preferably of the pillow-shaped type.

It is a particular advantage of the present invention that highlythermal shock-resistant agglomerates can be formed. Such agglomeratesare substantially dry and can be used in a process involvinggasification with oxygen, such as in a British Gas/Lurgi SlaggingGasifier, which is a high pressure, fixed bed, oxygen-blown, slagginggasifier, where fuel (lump coal or briquettes) is fed to the top of agasification bed via a lock hopper system. As the fuel sinks through thebed, it is successively dried, devolatilised, pyrolysed, gasified andcombusted. Oxygen and steam, in a suitable predetermined ratio, areinjected into the gasification zone via nozzles (called tuyeres), whichthereby ensures that the temperature is sufficiently high that ash andflux are melted so as to form a slag pool in the bottom of thegasification zone, which slag is drained intermittently and waterquenched to form a benign frit. Much of the detail of the BritishGas/Lurgi Slagging Gasifier is well known in the art, and is describedin, for example, GB patent 977122 and other literature.

The present invention will be illustrated, by way of example only, bythe following tabulated summary of results obtained with variousformulations formed into briquettes by a method according to theinvention.

Table 1 gives details of compositions of briquettes obtained accordingto the invention, together with the corresponding green and carbonisedcrush strengths of the briquettes (measured using a pneumatic briquettecrushing test rig). Each briquette tested contained calciumlignosulfonate (Ligno) as binder; the percentages and ratios quoted inthe column entitled "Composition of Organic Material" relate to theorganic material (that is, all the organic ingredients other than thelignosulfonate). The sludge used was sewage sludge which had been driedat at least 105 degrees Celsius to a residual moisture content of 1 to2.5% by weight. "Pitts. No 8 Coal" means Pittsburgh No. 8 Coal; "Pet.Coke" means petroleum coke.

In each case, the sludge and/or other organic material was mixed withthe aqueous calcium lignosulfonate, heated and oxidatively conditioned,and then shaped to pillow-shaped briquettes. "Lime before Ligno" meansthat calcium oxide was added before the lignosulfonate; "Lime afterLigno" means that calcium oxide was added after the lignosulfonate.

Table 2 gives details of the weight changes attained for individualbriquettes after they had undergone carbonisation in an oven in an inertatmosphere, where the temperature was increased from around ambient roomtemperature to about 800 degrees Celsius over a period of about onehour. The briquettes measured were the corresponding ones identified inTable 1.

                                      TABLE 1                                     __________________________________________________________________________                            Briquette Composition                                 Exam-                   Sludge                                                                            Pitts. No. 8             Crush Strength kgf       ple Composition of Organic Material                                                                   gm  Coal gm                                                                             Pet. Coke gm                                                                         Binder gm                                                                          Lime CaO gm                                                                          Green                                                                             Carbonised           __________________________________________________________________________    1   100% dried sludge   70  --    --     9    --     198 36                    2      100% dried sludge.    Lime before Ligno                                                                                                 27           3      100% dried sludge.    Lime after Ligno                                                                                                  45           4      100% Pitts No. 8 Coal                                                                                                                   *            5      100% Pitts No. 8 Coal.   Lime before Ligno                                                                                              *            6      100% Pitts No. 8 Coal.   Lime after Ligno                                                                                               *            7      100% Pet. Coke                                            122          8      2:1 Sludge/Pitts No. 8 Coal                                                                                                             115          9      2:1 Sludge/Pitts No. 8 Coal. Lime before Ligno                                                    44                                                                                                                  6871        10     2:1 Sludge/Pitts No. 8 Coal. Lime after Ligno                                                                                            101         11     1:1 Sludge/Pitts No. 8 Coal                                                                                                              117         12     1:1 Sludge/Pitts No. 8 Coal. Lime before Ligno                                                     36                                                                                                                  99          13     1:1 Sludge/Pitts No. 8 Coal. Lime after Ligno                                                                                            95          14     2:1 Sludge/Pet. Coke                                                                                                                     22          15     2:1 Sludge/Pet. Coke.   Lime before Ligno                                                                                                16          16     1:1 Sludge/Pet. Coke                                                                                                                     22          17     1:1 Sludge/Pet. Coke. Lime before Ligno                                __________________________________________________________________________                                                             20               

                                      TABLE 2                                     __________________________________________________________________________    Example                                                                            Composition         Initial Weight gm                                                                     Final Weight gm                                                                       Weight Loss gm                                                                        Weight Loss                  __________________________________________________________________________                                                     %                            1    100% dried sludge   42.7    17.0    25.7    59.3                         2          100% dried sludge. Lime before Ligno                                                                                                  58.5       3          100% dried sludge. Lime after Ligno                                                                                                   58.5       4          100% Pitts No. 8 Coal                                                                                                                    *       5          100% Pitts No. 8 Coal. Lime before Ligno                                                                                                 *       6          100% Pitts No. 8 Coal. Lime after Ligno                                                                                                  *       7          100% Pet. Coke                                                                                                                         14.5      8          2:1 Sludge/Pitts No. 8 Coal                                                                                                           50.5       9             2:1 Sludge/Pitts No. 8 Coal. Lime before Ligno                                                                                     49.6       10        2:1 Sludge/Pitts No. 8 Coal. Lime after Ligno                                                                                          50.1       11           1:1 Sludge/Pitts No. 8 Coal                                                                                                         46.9       12        1:1 Sludge/Pitts No. 8 Coal. Lime before Ligno                                                                                         45.1       13        1:1 Sludge/Pitts No. 8 Coal. Lime after Ligno                                                                                          44.2       14        2:1 Sludge/Pet. Coke                                                                                                                   46.4       15        2:1 Sludge/Pet. Coke. Lime before Ligno                                                                                                43.0       16        1:1 Sludge/Pet. Coke                                                                                                                   36.6       17        1:1 Sludge/Pet. Coke. Lime before Ligno                             __________________________________________________________________________                                                     33.2                     

We claim:
 1. A method of forming thermally shock-resistant chemicallybonded agglomerates suitable for use as a combustible fuel or in agasification process, which method comprises:(a) mixing particulateorganic material selected from the group consisting of a carbonaceousfuel and an animal excrement with a lignosulfonate, in the presence ofwater, with application of heat sufficient to maintain thelignosulfonate in fluid form, so as to deposit a film of lignosulfonateover the surface of the particulate material; (b) oxidativelyconditioning the lignosulfonate coated particulate material in anatmosphere comprising oxygen under conditions permitting escape of waterpresent in the mixed particulate material, said conditioning eitherbeing in the presence of calcium ions or being followed by mixing theconditioned particulate material with lime; and (c) shaping theresulting free-flowing plastic mixture under pressure, to formagglomerates.
 2. A method according to claim 1, wherein the oxygen is inthe form of air.
 3. A method according to claim 1, wherein the lime isin the form of the hydroxide and the lignosulfonate has a water contentof less than 5% by weight.
 4. A method according to claim 1, wherein thelignosulfonate is an ammonium, sodium, calcium or magnesium salt.
 5. Amethod according to claim 4, wherein the salt is calcium lignosulfonate.6. A method according to claim 1, wherein the lignosulfonate issubstantially sugar-free.
 7. A method according to claim 1, wherein thelignosulfonate is used in an aqueous form containing 40-60% dry weightof said lignosulfonate.
 8. A method according to claim 1, wherein theanimal excrement comprises dewatered sewage sludge.
 9. A methodaccording to claim 8, wherein the sewage sludge is dewatered and driedto a water content of not more than 10% by weight and is in the form offibres, flakes, powder or granules with a particle size of not more than10 mm.
 10. A method according to claim 1, wherein the weight ratio ofsaid animal excrement to said carbonaceous fuel is within the range 0.25to 2.5:1.
 11. A method according to claim 1, wherein the carbonaceousfuel comprises refuse-derived fuel, petroleum coke, anthracite,bituminous or sub-bituminous coal, coke from coal, coking coal, peat orlignite, all with a maximum particle size of 10 mm.
 12. A methodaccording to claim 1, wherein the mix is substantially free of municipalsolid waste.